Environmental Engineering Reference
In-Depth Information
However, millions of species have not even been identii ed let alone evalu-
ated for their potential values to humans. There are dii culties in predict-
ing which species will be important in the future since the present functions
performed by a species may provide no clues as to its role when environ-
mental conditions change (Main, 1982; Lovejoy, 1988). Tree species that
colonize gaps in tropical forests and species that require i re to enhance
germination provide such examples. The population of
Cactoblastis
cactorum
, which is relatively rare in Australia today, would not provide
an accurate description of its importance in controlling
Opuntia
in that
country in previous decades. However, studies on ecosystem function may
reveal clues as to the most sensitive components of food webs, and nutri-
ent and energy l ows. Research reveals that the most sensitive components
of ecosystems are those in which the number of species performing a
particular function is thought to be very small (Schindler, 1990).
Most ecologists recognize that some species play a more important
functional role than others. But what does this imply in terms of the prop-
erties of an ecosystem? Land managers are principally concerned with
ecosystem productivity and its variability. The next section provides a
review of the relationship between biodiversity and the stability, resilience
and productivity of ecosystems.
Importance of biodiversity in managed landscapes
The importance of biodiversity is associated with a controversial theoreti-
cal debate amongst ecologists that began in the early 1950s: does biodiver-
sity af ect the stability of an ecosystem? Holling (1973) refers to stability
as a characteristic of the individual populations of an ecosystem. For
example, stability is dei ned as the propensity of a population to return to
some kind of equilibrium following a disturbance. The stability of ecosys-
tems may be linked to their biodiversity and it has long been hypothesized
that more diverse ecosystems are more stable. A clue as to why this may
be the case is illustrated by a natural disturbance that deleted some species
from the ecosystem. A diverse system might be little af ected by the impact
because other species with similar niches could perform similar functions
to the missing species. Early advocates of this theory include MacArthur
(1955) who postulated that a highly diverse ecosystem would change less
upon the removal or addition of a species than would an ecosystem with
fewer species. Elton (1958) also suggested that less diversity resulted in less
ecological stability.
However these theories were not without their critics. May (1973)
challenged this argument and showed that a highly connected system
(higher biodiversity) may be less stable than simpler ones and more vul-
nerable to disturbance because all its components closely interact and are
